1. Field of the Invention
The present invention relates to a new improvement in a construction of a charge-coupled device (hereinafter referred to CCD), and especially relates to a construction for changing a transfer direction of charge signals.
2. Description of the Prior Art
Two different kinds of constructions of CCD are known for changing a transfer direction of charge signals. One of them uses a high conductive materials, more concretely a high impurity concentration diffused region, which connects two transfer channels having different transfer directions from each other. The article "Charge Transfer Device Analog Signal Processing" by David D. Wen, et al., 1976 IEEE International Solid-State Circuits Conference P. 204 and 205 shows such arts.
FIG. 1 shows a turning part of a CCD having the high conductive material for changing transfer directions of charge signals. In FIG. 1(a), a top view, A.sub.a1 and A.sub.a2 designate channels to transfer signal charges, and G designates a high impurity concentration region having high conductivity of other conductivity than a substrate Sb. The channels A.sub.a1 and A.sub.a2 are defined by very high impurity concentration layer Hic of the same conductivity type as the substrate Sb, serving for a so called channel stop, the end line of the layer is shown by I.sub.1 and it surrounds the channels A.sub.a1 and A.sub.a2. And E.sub.1 designates a turning pitch of the CCD. In FIG. 1(b), a sectional view taken on H-H' in FIG. 1(a), an insulating film J.sub.1 such as silicon oxide film is on the substrate Sb of the CCD. Electrodes C.sub.1 of stripe form are disposed on the insulating film J.sub.1 in a row and are respectively insulated by insulating films J.sub.2. Electrodes B.sub.1 are disposed between each gap of two neighboring insulating films J.sub.2 as shown in FIG. 1(b). This CCD is so called a two-phase CCD which has high impurity concentration layers K.sub.1 under the respective electrodes B.sub.1. In this kind of CCD, the adjoining electrodes B.sub.1 and C.sub.1 are connected each other by contacts D.sub.1. The contacts D.sub.1 are also connected with every other ones each other.
In this CCD, signal charges are transferred by clock pulses applied to two kinds of electrodes B.sub.1 and C.sub.1 in the channel A.sub.a1 in the direction shown by arrow F.sub.a1. When signal charges reach the diffused region G, the transfer direction of signal charges turns from the arrow F.sub.a1 to an arrow F.sub.a2 by the high conductive diffused region G, and then the signal charges are transferred along the channel A.sub.a2 leftwards of FIG. 1(a). Since the high conductive diffused region G can be arbitrarily changed in almost all desired form, the turning part of the transfer direction can be made a simple structure, and thereby a surface area utilization rate in an IC chip (namely, a rate of a part in which signal charges and current pass through and directly contributes to the operation of CCD to whole surface of an IC chip) can be increased.
However, because the diffused region G is in the channel this CCD has the following disadvantages: First, a speed which signal charges are injected again into the channel A.sub.a2 is limited by a time constant .tau., which is a ratio of a stray capacity of the diffused region G against the earth to a channel conductance of a storage site St where signal charges are injected again from the diffused region G. Therefore if the time constant .tau. is larger than a value .tau.e determined by transfer frequency of the CCD, the turning parts worsen the frequency characteristics of the CCD. To minimize the stray capacity of the diffused region G for reducing the time constant .tau. is limited by a pattern design of the CCD. And if an effective electrode width of the electrode C.sub.A1 namely the width of the storage site St is shortened in order to enlarge the channel conductance, it produces so called a two dimensional effect, that is, modulation of a voltage of the storage site St by an amount of signal charges, thereby the transfer efficiency of the CCD becomes worse. As described above, the turning method for changing transfer direction has the same disadvantage as that of the MOS type bucket brigade device as the frequency characteristics.
Secondly a dark current of the diffused region G is larger than that of the ordinary channel part. Since the diffused region G is in itself inserted into the two neighboring electrodes in the channel, the dark current of the diffused region is added to the dark current of one transferring stage and that of the diffused region G, and amount to several times as large as that of one transferring stage. Such characteristic becomes a serious problem especially when the CCD is used, for example, as a temporary memory device whose driving by clock pulse is temporarily halted and from which memorized data in the CCD are read after a short time. If the dark current is generated in the CCD uniformly and entirely, only a DC voltage based on the dark current is added to an output signal of the CCD. However, in such CCD, since the dark current of diffused region G is larger than every other channel, a fixed pattern noise is added to the output signal, and thereby a noise characteristic of the CCD becomes very worse. Therefore, nowadays such method using the diffused region is hardly used.
Other prior art is mentioned below. FIG. 2 shows a construction of CCD to change a transfer direction of charge signals. In this CCD, symbols A.sub.b, B.sub.2, C.sub.2, D.sub.2, E.sub.2 and I.sub.2 correspond respectively to the counterparts of FIG. 1, the channel A.sub.a, the electrode B.sub.1, the electrode C.sub.1, the contact D.sub.1, the turning pitch E.sub.1 and the end line of channel stop I.sub.1. And a symbol L shows a length correspond to one bit of the CCD. As illustrated in FIG. 2, the electrodes B.sub.2 and C.sub.2 and channel A.sub.b are arranged in a shape of a semi-circle, and the transfer direction of signal charges is changed from an arrow F.sub.b1 to an arrow F.sub.b2. In the CCD of this type, since a considerable area which can not be used is retained in the center of the circle, a surface area utilization rate in an IC chip is low. The reason is further described below in detail referring to FIG. 3.
Providing:
R; outer radius of the channel A.sub.b
r; inner radius of the channel A.sub.b
A.sub.w ; width of the channel A.sub.b
.theta.; angle between both side lines of the electrode C.sub.2, the following relations hold. EQU R-r=A.sub.w EQU L.sub.o .congruent.R.multidot..theta.
(L.sub.o ; outer length of the electrode within the channel A.sub.b) EQU L.sub.i .congruent.r.multidot..theta.
(L.sub.i ; inner length of the electrode within the channel A.sub.b)
By the way, a minimum length L min. of electrode is determined as follows by a minimum dimension obtainable in IC technology: L.sub.i .gtoreq.L min.
Furthermore, the length of electrode should not be lengthen as far as a maximum length L max. so as not to worsen the transfer characteristic by increasing charge transfer time. Therefore EQU L.sub.o .ltoreq.L max.
has to be held.
To sum up the abovementioned inquality, the following relation holds: ##EQU1##
Therefore the inner radius r has to be a length excess to a specified value, an invalid and useless area is needed. For example, in case L max. is 14 (.mu.m) L min. is 6 (.mu.m), and A.sub.w is 40 (.mu.m), r has to be at least 30 (.mu.m). When changing the transfer direction of signal charges from the arrow F.sub.b1 to F.sub.b2 as shown in FIG. 2, such a large semi-circle having a diameter of 60 (.mu.m) is required, and it wastes the surface area of the IC chip.
Furthermore, it takes a long time to make a pattern of a semi-circle shape, especially a photo mask for IC.
For the explanations mentioned above, conventional method has had various kinds of defects.